The crystal structure of hydrated sodium aluminate, (\mathrm{NaAlO}{2}). (5 / 4 \mathrm{H}{2} \mathrm{O}), has been determined (a b) initio from synchrotron powder diffraction data. The compound crystallizes in the acentric primitive tetragonal space group (\bar{P} 2_{1} m), with (a=10.53396(4) \AA) and (c=) (5.33635(3) X) at (27^{\circ} \mathrm{C}), and (Z=8). The structure consists of single layers parallel to (001) made of corner-sharing (\mathrm{AlO}{4}) tetrahedra, joined into 4- and 8-rings. The topology of the layers is that of framework 18 of Smith and Rinaldi (14). The layers are held together by two types of (\mathrm{Na}) : one 6-coordinate, with nearly ideal octahedral geometry, and the second 7-coordinate, with a capped octahedral coordination. Two independent water molecules also occupy the interlayer regions, forming hydrogen bonds to framework (\mathrm{O}) atoms. One oxygen of each (\mathrm{AlO}{4}) coordination sphere is a hydroxyl group, which points into the interlayer region. Dehydration of (\mathrm{NaAlO}{2}). (5 / 4 \mathrm{H}{2} \mathrm{O}) causes a structural transformation to (\mathrm{NaAlO}{2}), surprisingly involving no intermediate phases. The crystal structure of dehydrated (\mathrm{NaAlO}{2}) was refined by the Rietveld method using laboratory powder X-ray data. This compound crystallizes in (P n a 2_{1}), with (a=) (5.3871(2) \AA, b=7.0320(2) \AA), and (c=5.2180(2) \AA) at (25^{\circ} \mathrm{C}, Z=4). The structure is based on a three-dimensional framework of cornerlinked (\mathrm{AIO}{4}) tetrahedra and is isostructural to (\beta-\mathrm{NaFeO}{2} . \mathrm{NaAlO}{2}) can be rehydrated to form (\mathrm{NaAlO}{2} \cdot 5 / 4 \mathrm{H}_{2} \mathrm{O}); the dehydration process, which is accompanied by large changes in framework topology, is reversible. 1995 Academic Press, Inc.